Around here we usually stick pretty strictly to the idea of matching sticks. However many there are they should all be identical.
There is, however, no super good reason to stick strictly to that. Laptops commonly mix and they work well enough for the millions of people that use them.
It may not be... optimal... that way, but usable and useful it is. As un-traditional as it is, 6 is still better than 4.
If one stick was wildly slower than the other, as if one was 800 mhz and the other was 1600 mhz, then it would be a good idea to take out the slower one because the computer makes both equal to the slowest.
Otherwise, it is fine to mix even if it makes us cringe.
Per your question to tryanthboth, I don't think it really indicates a problem. You can probably ignore it pretty safely until such a time in the future that it actually affects how you use the computer. If at some point the monitor doesn't come back on afterwards, that would be a good time to worry.